1. Field of the Invention
The present invention relates to an optical displacement sensor and more particularly relates to improvement of an optical displacement sensor which detects target objects on the basis of the output of a light reception device which receives light reflected by the target objects when projected light is directed to the target objects.
2. Description of the Related Art
As sensors for detecting the presence or absence of a target object by directing light to the target object, there have been known photoelectric sensors and optical displacement sensors. Such photoelectric sensors are sensors which determine the presence or absence of a target object, on the basis of whether or not a light reception device receives projected light emitted from a light projection device (for example, Japanese Published Patent No. 2006-80896). Optical displacement sensors are sensors which calculate the amount of displacement of a target object, on the basis of light reflected by the target object when projected light is directed to the target object, and determine the presence or absence of the target object, on the basis of the calculated an amount of displacement.
Such optical displacement sensors are capable of determining the presence or absence of objects without being influenced by the colors and the materials of the objects, in comparison with photoelectric sensors which perform the determination simply on the basis of the amount of received light, since the amount of received light detected by the light reception device is varied depending on the colors and the materials of the target objects. Further, a displacement sensor including a light reception device constituted by a plurality of light reception elements arranged in a linear shape for receiving light reflected by a target object determines a one-dimensional position of the received-light spot on the light reception device, on the basis of the amount of light received by the respective light reception elements. The amount of displacement of a target object is calculated on the basis of the determination of position of the received-light spot obtained in such a manner.
FIG. 16 is a view illustrating the structure of a conventional optical displacement sensor 100, schematically illustrating a state where projected light emitted from a light projection device 101 is received by a light reception unit 104. The optical displacement sensor 100 is constituted by the light projection device 101, and the light reception unit 104 constituted by a light reception lens 102 and a light reception device 103. The light projection device 101 is a light source device for directing projected light to a work (target object) 120 placed on a work bench 110. The light reception lens 102 is a condenser lens for converging the light reflected by the work 120 from the projected light on the light reception device 103. The light reception device 103 is a light reception device for receiving the reflected light from the work 120 and outputs signals corresponding to the position of the received-light spot. The light emitted from the light projection device 101 and directed to the work 120 is reflected by the work 120 and is converged at a position on the light reception device 103 which is varied depending on the height of the irradiation point on the work 120, namely the position of the irradiation point in the direction of the optical axis of the projected light. The displacement sensor 100 determines the one-dimensional position of the received-light spot on the light reception device 103 on the basis of the output of the light reception device 103 and calculates the amount of displacement of the work 120 on the basis of the result of the determination.
FIG. 17 is a view illustrating the optical displacement sensor 100 placed in a manufacturing line. The displacement sensor 100 is used for detecting the presence or absence of works 120 being transferred on the work bench 110 in the direction of the line. In this case, when a work 120 exists at a position beneath the displacement sensor 100 to which the projected light is directed, the position of the received-light spot is different from the position of the received-light spot of when a work 120 does not exist. The displacement sensor 100 detects the presence or absence of the work 120, utilizing the fact that the position of the received-light spot is varied depending on whether or not the work 120 exists, and the amount of displacement of the work 120 is changed.
FIG. 18 is a timing chart illustrating operations of the optical displacement sensor 100 for detection of works, illustrating detected values of the amount of displacements and sensor outputs indicative of the presence or absence of works 120. The detected value of the amount of displacement is the position of the irradiation point in the direction of the optical axis of the projected light and is calculated on the basis of the position of the received-light spot. The detected value x200 of the amount of displacement acquired when a work 120 exists is greater than the detected value x100 of when the work 120 does not exist. The threshold value u100 is a threshold value for use in determining the presence or absence of a work 120 and is preliminarily determined according to the thicknesses of the works in the direction of the optical axis of the projected light (x100<u100<x200). If the detected value of the amount of displacement is decreased to below the threshold value u100, the sensor output is turned off, and the voltage level thereof is switched from a high level to a low level. Further, if the detected value of the amount of displacement is increased to above the threshold value u100, the sensor output is turned on, and the voltage level thereof is switched from the low level to the high level. The displacement sensor 100 determines the presence or absence of a work 120, by making a comparison between the detected value of the amount of displacement and the threshold value u100, as described above.